A method and a device of the detailed manner are described in CH 553 588. There a filter drum already positioned completely below the liquid level is used, the cylindrical surface of which is coated with a filter medium, the liquid being passed through the filter drum from the outside to the inside. The whole circumference of the cylindrical surface is used for filtering purpose. A backwashing device is provided for the removal of the filter cake from the filter medium, the divice containing a suction head in shape of a suction bar located in the lower region of the filter drum on the outside, making it possible to withdraw the filter cake in intervals. It is a disadvantage that the cleaning effect of the filter medium is limited by the performance of the pump of the suction device. Due to the limited performance there often is the danger that the filter medium, which may be a piece of fabric, gets plugged in such a manner so that an effective cleaning action in regulary con intervals is necessary, the cleaning action being performed by fluid ejecting jets under high pressure acting on the filter medium. In addition, by this plugging action the cycles of filtration are shortened. During cleaning by the suction device, the filter medium, in the shape of a piece of fabric, is sucked on with the result that during the lifetime of the fabric it is stretched and worn by the suction head of the suction device.
From U.S. Pat. No. 5,139,670 a device for filtering and backwashing solid particles out of liquids is known, in which a filter drum is used, the dirty liquid is directed to flow from the inside to the outside of the filter drum. Disadvantageously, not the whole circumference of the cylindrical surface of the filter drum may be used at the same time. There is provided a backwashing device containing a spraying head of an ejection device located at the outside of the filter drum and above the liquid level. Thus, for cleaning purposes, it is possible to spray liquid jets to the filter medium of the filter drum, resulting in a removal of the filter cake at the inner surface of the filter drum, the filter cake being withdrawn by a hopper and a conduit connected to it and leading out of the filter drum. The resulting cleaning effect is good, because the sprayed fluid jets have to penetrate a room filled with air and show their action without decrease.
In EP 74 966 B1 a device for filtering solid particles out of a dirty liquid is described, especially sewage in a sewage treatment plant, in which no filter drum but rather flat filter plates coated with a filter medium are used. A backwash device comprising a spraying device with a spraying head on the one side of the filter medium and a suction device with a suction head on the other side of the filter medium is provided. The spraying head and the suction head are also located in contact with the filter plate or the filter medium resulting in a sealing action preventing the suction of the wrong liquid. It is a disadvantage of this device that the filter plates with the sealing elements make the device complicated and costly. Due to the contact between the filter plates and the filter medium with the spraying head and the suction head also, there is strong friction and wear of the filter plates and of the filter medium, which shortens their lifetime. The effect of the cleaning action is limited in principle, because the sprayed liquid jets have to penetrate a liquid cushion and are slowed rapidly prior to reaching the filter medium. When the cleaning capacity is overridden at such a device, then the filter medium is plugged more and more resulting in a raising of the liquid level. Disadvantageously, the difference in pressure increase results in a clamping action between the plugged filter medium and the elements of the cleaning device, namely the spraying head and the suction head, moved with respect to the filter plates. Thus the backwashing device cannot meet the expected functions.